Error types are not Send & Sync

[pragmatrix]

Hi,

I'd like to use the realtime API and the first roadblock is that the error types are not Send + Sync, which breaks compatibility with anyhow. I'll wrap the errors for now, but thought you should know. Tell me if you'd like me to provide a PR or if you plan to introduce custom errors anyway.

[dongri]

@pragmatrix Thank you!
Please give me a pull request. Thanks!